1. Field of the Invention
The present invention relates to an aluminium brazing paste to be used for joining aluminium-made devices to one another. The term "aluminium-made" used herein is to be understood to include various aluminium alloys unless otherwise specified.
2. Description of the Prior Art
As the method for brazing aluminium-made devices to one another using metal powder, a method has been employed that the aluminium oxide film present on the surface of the area to be joined is removed using flux, and then brazed. In this method, since the resin to be added as a vehicle or a stickiness-improving agent (for example carboxy-methylcellulose, rosins and vinyl acetate resin) is separated and/or carbonized in the course of raising temperature to the brazing temperature (about 600.degree. C.), voids or black residues tend to occur in the fillet to cause defective appearance and defective joining. It is also noted that formation of reactive pyrolytic products often causes deterioration in the performance of the aluminium brazing paste and other problems.
For the sake of solving these problems it has been known that an organic resin that is volatilized away instead of being decomposed or carbonized during the heating process to the brazing temperature is formulated to the metal powder for brazing as a vehicle or a binder. As the resins for such a purpose, for example, acrylic resins are disclosed in Japanese patent laid-open publication No. 147193/1990; ethylene oxide polymers, polyacrylate polymers, polymethacrylate polymers, polyacrylonitrile polymers, polyethyleneglycols and their methyl ethers and aliphatic hydrocarbon polymers are described in Japanese patent publication No. 43200/1988; and polyisobutylene and polybutene are disclosed in Japanese patent laid-open publication No. 268995/1990.
However, in the invention disclosed in Japanese patent laid-open publication No. 147193/1990 toxic monomers with stimulant odor occur by pyrolytic depolymerization of acrylic resins posing problems in operation hygiene while in the one disclosed in Japanese patent publication No. 43200/1988, in which fluorides of alkali metal are employed together with highly hygroscopic chlorides as flux, washing process is required after brazing for prevention of corrosion of aluminium due to the action of the chlorides; in the disclosure in Japanese patent laid-open publication No. 268995/1990 there is no description on the aluminium brazing with an aluminium powder that poses such a particular problem as difficulty of obtaining sufficient joining strength due to prevention of flowing out of the metal molten within the powder at brazing since strong oxide film is present on the surface of the powder, and accordingly it is not clear whether the binder for the paste disclosed in the invention, namely the binder made by formulating polyisobutylene or other polymers with a definite amount of specific fluidity depressant, is effective as an ingredient of the aluminium brazing paste, and in addition no detailed description is given on flux. Cellulose-type resins and many polymers do not allow melting of the brazing metal powder upon heating often remaining metal residue which contain unmolten metal powders.
On the other hand, fluorine-type flux has been proposed as the flux to be used in aluminium brazing, for improving the above defects of chloride-type flux (see, for example, Japanese patent laid-open publication No. 123749/1976 and Japanese patent publications No. 27037/1983 and No. 46280/1987). In the inventions described in these publications an aqueous slurry made by dispersing water-insoluble, fluorine-type flux together with metal powder for brazing in water is made adhered onto the part to be joined, and after evaporating water off therefrom brazing is carried out, with the result of requiring additional heating process for the evaporation of water. There also is another problem of ready variation of viscosity along with time. It must also be pointed out that use of metal powder with a very large surface area requires a large amount of flux often causing pollution and corrosion of furnace wall and aluminium-made devices.